While electromagnetic interactions were first used to probe the structure of elementary particles more than 20 years ago, their importance has only become fully evident in the last 10 years. In the resonance region, photo production experiments have provided clear evidence for simple quark model ideas, and confirmed the Melosh-transformed SU(6)w as a relevant symmetry classification. At higher energies, their most striking feature is their similarity to hadron-induced reactions, and they have provided fresh insight into the ideas developed to explain strong-interaction physics. New dimensions are added by taking the photon off mass shell, both in the spacelike region, where the development of high-energy electron and muon beams has led to the discovery and study of scaling and the intro duction of "partons," and even more dramatically in the timelike region, where the development of high-energy electron-positron storage rings has led to the exciting discoveries of the last four years. In view of the immense interest stimulated by these developments, an extensive review of our present state of knowledge is both timely and useful. Because of the very wide range of the subject, a cooperative venture presents itself as the most suitable format and is the one we have adopted here. The emphasis throughout is primarily, but not entirely, on phenomenology, concentrating on describing the main features of the experimental data and on the theoretical ideas used directly in their inter pretation.

This book consists of a collection of carefully selected review papers and lecture notes written by the author on Perturbative Quantum Chromodynamics (PQCD) and its experimental tests. A leading authority who has contributed significantly to the development of PQCD for more than 20 years, the author has selected articles on the basis of their present interest. These articles cover different aspects of the subject without unnecessary duplication and are accompanied by commentaries. Also, original material, specially written for this book, has been added to integrate and update the results and their presentation. Hence, more than providing a historical perspective on the subject, this volume also serves well as an authoritative guidebook on Perturbative QCD.